The correct answer is C) manufacturer's numerical code. The size of ball and roller bearings is identified by the manufacturer's numerical code, which is a standardized system that provides information about the bearing's dimensions and specifications. This code is typically printed on the bearing or its packaging, and it allows users to quickly determine the appropriate bearing for their application. The other answer choices are incorrect because they do not directly indicate the size of the bearing. The rolling member size (A) and the outer ring width (D) are specific dimensions of the bearing, but they do not uniquely identify the overall bearing size. The inner race cone width (B) is also a specific dimension, but it is not the primary means of identifying the bearing size.